In the development of semiconductor devices, particularly those for use in integrated circuits, it is frequently desirable to obtain a structure wherein two independent regions of the same conductivity type overlying a region of opposite conductivity type, are separated by a thick oxide filled region called an oxide moat. Thus, for example, two n-type regions would be separated by an oxide moat overlying a p-type substrate. The oxide moat is ordinarily prepared by the selective oxidation of the semiconductor and is carried to such a depth that its bottom surface intersects the substrate. Because the solubility of impurities in the oxide and in the semiconductor are different, there is, during the oxidation process, a tendency for the impurity concentration in the semiconductor adjacent to the growing oxide region to be either depleted or enhanced. Thus, real structures differ significantly from ideal structures in terms of the impurity concentrations in the doped regions surrounding the oxide moat. In the example mentioned earlier, boron is typically depleted from the p-type substrate beneath the oxide moat so that an approximately intrinsic region is formed. The built-in charge in the oxide typically causes the intrinsic region to form a parasitic n-channel connecting the two n-type regions on either side of the moat, thus interfering with proper operation of the device or circuit.
In the prior art, various techniques have been utilized to overcome this problem, the most common being to dope the moat region prior to oxidation. For example, using p-doping one can overcome depletion effects in a p-region underlying an oxide moat. However, at the same time, this approach creates a p-skin in the n-regions which form the sidewalls of the oxide moat, thus creating a new set of problems which are frequently as intolerable as the original problem. Additionally, it is generally desirable to have a process for producing buried doped regions of arbitrary conductivity and type (e.g., channels under an epitaxial layer) which self-align with other abutting buried regions.
Thus a need continues to exist for a fabrication process which can locally dope a semiconductor region underlying an oxide moat or an epitaxial layer so as to overcome either enhancement or depletion effects associated with oxidation and which does not result in the formation of undesirable parasitic channels at other locations, or which can provide channels of arbitrary conductivity and type buried within the structure, that are self-aligned with and abutting other doped regions.
Therefore, it is an object of this invention to provide an improved process for forming in a semiconductor wafer a selectively doped buried region of a second dopant abutting a region of a first dopant.
It is an additional object of this invention to provide an improved process for forming a selectively doped buried region underlying an oxide moat in a semiconductor structure.
It is a further object of this invention to provide an improved process for forming a selectively doped buried region without creation of parasitic doped regions in other locations in the semiconductor structure. It is an additional object of this invention to provide an improved process for forming a selectively doped buried region, which is self-aligning, which does not require additional masking steps, and which facilitates subsequent alignment steps.